SRAM cell with balanced write port

ABSTRACT

A semiconductor device includes first, second, third, and fourth active regions arranged along a first direction. The first, second, third, and fourth active regions includes channel regions and source/drain (S/D) regions of first, second, third, and fourth transistors respectively, the first and fourth transistors are of a first conductivity type, and the second and third transistors are of a second conductivity type opposite the first conductivity type. The semiconductor device further includes a fifth active region between the second and third active regions. The fifth active region includes channel regions and S/D regions of fifth and sixth transistors that are of same conductivity type. The semiconductor device further includes first, second, third, fourth, fifth, and sixth gates. The first through sixth gates are disposed over the channel regions of the first through sixth transistors respectively. The first, second, and fifth gates are electrically connected.

PRIORITY

This is a divisional of U.S. patent application Ser. No. 15/625,490,filed Jun. 16, 2017, herein incorporated by reference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. In the course of IC evolution, functional density (i.e., thenumber of interconnected devices per chip area) has generally increasedwhile geometry size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling downprocess generally provides benefits by increasing production efficiencyand lowering associated costs. Such scaling down has also increased thecomplexity of processing and manufacturing ICs and, for these advancesto be realized, similar developments in IC manufacturing are needed.

For example, as the scaling down continues, conventional 6T(6-transistor) static random access memory (SRAM) cell suffers fromstability problems during read and write operations, where the cell isvulnerable towards noise. To overcome such issue, 8T (8-transistor) SRAMcell designs have been proposed, where write port (write word/bit lineswith 6 transistors) are separate from read port (read word/bit lineswith 2 transistors). However, existing 8T SRAM cell is not completelysatisfactory. For example, the 6 transistors in the write port inconventional 8T SRAM cells are often unbalanced or asymmetrical, whichoften leads to increased Vccmin (minimum operation voltage). IncreasedVccmin leads to increased power consumption and hence is not desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when they are read with the accompanying figures.It is noted that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1A shows a logic diagram of a SRAM cell, according to aspects ofthe present disclosure.

FIG. 1B shows a layout design of and a top view of the SRAM cell of FIG.1A, in accordance with some embodiments.

FIG. 1C illustrates a cross-sectional view of the SRAM cell of FIG. 1Balong the A-A′ line of FIG. 1B, in accordance with an embodiment.

FIG. 1D illustrates a cross-sectional view of the SRAM cell of FIG. 1Balong the A-A′ line of FIG. 1B, in accordance with another embodiment.

FIG. 1E illustrates a cross-sectional view of the SRAM cell of FIG. 1Balong the B-B′ line of FIG. 1B, in accordance with an embodiment.

FIG. 1F illustrates a cross-sectional view of the SRAM cell of FIG. 1Balong the B-B′ line of FIG. 1B, in accordance with another embodiment.

FIG. 1G illustrates a cross-sectional view of the SRAM cell of FIG. 1Balong the B-B′ line of FIG. 1B, in accordance with yet anotherembodiment.

FIG. 1H illustrates a cross-sectional view of the SRAM cell of FIG. 1Balong the B-B′ line of FIG. 1B, in accordance with an embodiment.

FIG. 2A shows a layout design of and a top view of another SRAM cell,according to various aspects of the present disclosure.

FIG. 2B shows the logic diagram of the SRAM cell of FIG. 2A in anembodiment.

FIG. 3A shows a layout design of and a top view of yet another SRAMcell, according to various aspects of the present disclosure.

FIG. 3B shows the logic diagram of the SRAM cell of FIG. 3A in anembodiment.

FIG. 4A shows a layout design of and a top view of an SRAM cell,according to various aspects of the present disclosure.

FIG. 4B illustrates a perspective view of some connectivity of the SRAMcell of FIG. 4A, in accordance with some embodiment.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present application is generally related to SRAM cell designs, moreparticularly to 8T SRAM cell designs with a symmetrical write portlayout. Features of the present disclosure can be applied to SRAMdesigns with CMOS (complementary metal-oxide-semiconductor) planar FET(field effect transistor) or multi-gate FET devices includingdouble-gate FET, triple-gate FET, omega-gate FET, and gate-all-around(or surround-gate), and/or FinFET (field effect transistor with fin-likechannels).

FIG. 1A shows a schematic logic diagram of a SRAM cell 100 according toaspects of the present disclosure. FIG. 1B shows a layout (of certainlayers) top view of the SRAM cell 100 in an embodiment. Referring toFIG. 1A, the SRAM cell 100 includes a write port and a read port. Thewrite port includes two inverters cross-coupled for storage. The firstinverter includes a pull-up transistor PU1 (or Tr2) and a pull-downtransistor PD1 (or Tr1) connected in series between high and lowpotentials, VDD1 and VSS1. The second inverter includes a pull-uptransistor PU2 (or Tr3) and a pull-down transistor PD2 (or Tr4)connected in series between the high and low potentials, VDD1 and VSS1.The write port further includes two pass gate transistors PG1 (or Tr7)and PG2 (or Tr8). The gate terminals of PG1 and PG2 are connected toword line WL. One of the two source/drain (S/D) terminals of PG1 iscoupled to the gate terminals of PU2 and PD2, and the other one of thetwo S/D terminals of PG1 is coupled to bit line BL. One of the twosource/drain (S/D) terminals of PG2 is coupled to the gate terminals ofPU1 and PD1, and the other one of the two S/D terminals of PG2 iscoupled to inverse bit line (BLB). The read port includes twotransistors Tr5 and Tr6. In the embodiment shown, the gate terminal ofTr5 is coupled to the gate terminals of PU1 and PD1. One of two S/Dterminals of Tr5 is coupled to a low potential VSS2 and the other one iscoupled to one of two S/D terminals of Tr6. The other S/D terminal ofTr6 is coupled to read bit line RBL. The gate terminal of Tr6 is coupledto read word line RWL. Since the read port is separate from the writeport, the 8T SRAM cell 100 has better noise immunity than conventional6T SRAM cells.

Referring to FIG. 1B, the transistors Tr1 through Tr8 of the SRAM cell100 are formed over various active regions 102, 104, 106, 108, and 110.Particularly, the active regions 102, 104, 106, 108, and 110 areoriented lengthwise along the “y” direction and are arranged in orderfrom first to fifth along the “x” direction. The transistors Tr1 throughTr8 further includes gates (or gate stacks or gate terminals) G1, G2,G3, G4, G5, G6, G7, and G8, respectively. The active regions 102, 104,106, 108, and 110 may be in the form of planar active regions, where therespective gate is disposed over a flat surface of the respective activeregion. Alternatively, the active regions 102, 104, 106, 108, and 110may be in the form of active fins, where the respective gate is disposedover two or more surfaces of the respective active fin, making thetransistors Tr1 through Tr8 FinFETs.

Still referring to FIG. 1B, the active region 102 comprises channelregions and S/D regions of the transistors Tr1 and Tr7. The channelregions of Tr1 and Tr7 are underneath the gates G1 and G7 respectively,and the S/D regions of Tr1 and Tr7 are on opposite sides of the gates G1and G7 respectively. In the present embodiment, Tr1 and Tr7 share an S/Dregion that is between the gates G1 and G7. In an alternativeembodiment, Tr1 and Tr7 have separate S/D regions.

The active region 104 comprises a channel region and two S/D regions ofthe transistor Tr2. The channel region of Tr2 is underneath the gate G2,and the S/D regions of Tr2 are on opposite sides of the gate G2. Theactive region 106 comprises a channel region and two S/D regions of thetransistor Tr3. The channel region of Tr3 is underneath the gate G3, andthe S/D regions of Tr3 are on opposite sides of the gate G3.

The active region 108 comprises channel regions and S/D regions of thetransistors Tr4 and Tr8. The channel regions of Tr4 and Tr8 areunderneath the gates G4 and G8, respectively, and the S/D regions of Tr4and Tr8 are on opposite sides of the gates G4 and G8, respectively. Inthe present embodiment, Tr4 and Tr8 share an S/D region that is betweenthe gates G4 and G8. In an alternative embodiment, Tr4 and Tr8 haveseparate S/D regions.

The active region 110 comprises channel regions and S/D regions of thetransistors Tr5 and Tr6. The channel regions of Tr5 and Tr6 areunderneath the gates G5 and G6, respectively, and the S/D regions of Tr5and Tr6 are on opposite sides of the gates G5 and G6, respectively. Inthe present embodiment, Tr5 and Tr6 share an S/D region that is betweenthe gates G5 and G6. In an alternative embodiment, Tr5 and Tr6 haveseparate S/D regions.

Each of the active regions 102, 104, 106, 108, and 110 comprises one ormore semiconductor materials such as silicon, germanium, silicongermanium, silicon carbide, gallium arsenide, indium arsenide, indiumphosphide, GaAsP, AlInAs, AlGaAs, InGaAs, GaInP, and/or GaInAsP, orcombinations thereof.

The channel regions of the transistors Tr1 through Tr8 may be doped orundoped (including unintentionally doped). The S/D regions of thetransistors Tr1 through Tr8 are doped with appropriate materials for theconductivity type of the respective transistor. In an embodiment, thetransistors Tr2 and Tr3 are PMOS FETs (p-type conductivity) and theother transistors, Tr1 and Tr4 through Tr8, are NMOS FETs (n-typeconductivity). Therefore, the S/D regions of the transistors Tr2 and Tr3are doped with a p-type material such as boron, and the S/D regions ofthe other transistors are doped with an n-type material such asphosphorus. The S/D regions of the transistors Tr1 through Tr8 maycomprise epitaxially grown semiconductor material, such as epitaxiallygrown silicon for the NMOS FETs or epitaxially grown silicon germaniumfor the PMOS FETs.

The gates G1, G2, G3, G4, G5, G6, G7, and G8 are oriented lengthwisealong the “x” direction. In the present embodiment, the gates G1, G2,G8, and G6 are aligned on a straight line; and the gates G7, G3, G4, andthe G5 are aligned on another straight line. Each of the gates G1through G8 includes a gate dielectric layer and a gate electrode layerover the gate dielectric layer. In some embodiments, each of the gatesG1 through G8 may further include an interfacial layer between the gatedielectric layer and the underlying channel semiconductor material. Thegate electrode layer in the gates G1 through G8 may include one or morework function layers and a metal fill (or bulk metal) layer. The gatesG1 and G2 are electrically connected, for example, by sharing a commonmetal layer in the respective gates in the embodiment shown or by upperlevel metal interconnects in an alternative embodiment. The gates G3 andG4 are electrically connected, for example, by sharing a common metallayer in the respective gates in the embodiment shown or by upper levelmetal interconnects in an alternative embodiment.

The SRAM cell 100 further includes various contact (or S/D contact)features 122, 124, 126, 128, 136, 138, 140, 142, 144, and 146 disposedover the S/D regions of the transistors Tr1 through Tr8. The contact 122is disposed over the shared S/D region of Tr1 and Tr7. The contacts 124and 140 are disposed over the other S/D regions of Tr1 and Tr7respectively. The contact 124 serves as one VSS1 terminal. The contact140 serves as the BL terminal. The contact 122 is also disposed over anS/D region of the transistor Tr2 to electrically couple the S/D regionsof Tr1, Tr2, and Tr7. The contact 126 is disposed over another S/Dregion of Tr2 and serves as one VDD1 terminal.

The contact 128 is disposed over the shared S/D region of Tr4 and Tr8.The contacts 146 and 142 are disposed over the other S/D regions of Tr4and Tr8 respectively. The contact 142 serves as the BLB terminal. Thecontact 146 serves as one VSS1 terminal. The contact 128 is alsodisposed over an S/D region of Tr3 to electrically couple the S/Dregions of Tr3, Tr4, and Tr8. The contact 144 is disposed over anotherS/D region of Tr3 and serves as one VDD1 terminal.

The contact 136 is disposed over an S/D region of Tr5 and serves as theVSS2 terminal. The contact 138 is disposed over an S/D region of Tr6 andserves as the RBL terminal.

The SRAM cell 100 further includes various conductive features 130, 132,and 134. The conductive feature 130 electrically connects the S/Dcontact 122 and the gate G3. The conductive feature 132 electricallyconnects the S/D contact 128 and the gate G2. The conductive feature 134electrically connects the S/D contact 128 and the gate G5. Theconductive features 130, 132, and 134 may include one or more elementalmetals, metal alloy, conductive metal oxide, conductive metal nitride,or other suitable conductive materials. Effectively, the S/D regions ofthe transistors Tr1, Tr2, and Tr7 and the gates G3 and G4 areelectrically connected; and the gates G1, G2, and G5 and the S/D regionsof the transistors Tr3, Tr4, and Tr8 are electrically connected.

The SRAM cell 100 further includes one or more insulating materials 112and 114 to electrically isolate various components. Particularly, theinsulating material 112 is disposed between the gates G4 and G5 toelectrically isolate the two. The insulating materials 112 and 114 mayinclude silicon oxide, silicon nitride, silicon oxynitride, a low-kdielectric material, or other suitable dielectric material(s). Theinsulating material 112 and the insulating material 114 may comprise thesame or different dielectric materials.

In various embodiments, the transistors in the read port (Tr5 and Tr6)and the transistors in the write port (Tr1 through Tr4, Tr7, and Tr8)are designed to have different threshold voltages (Vt). For example, thetransistors Tr1 through Tr4 may be designed to have standard Vt whilethe transistors Tr5 and Tr6 are designed to have low Vt or ultra-low Vt(lower than the standard Vt) to speed up the read operations. Manyfactors affect the threshold voltage of a transistor, one of which isthe work function of the gate of the transistor. Oftentimes, a gate canbe designed with appropriate work function layer(s) to provideappropriate threshold voltage of the transistor. For example, eventhough transistors Tr4 and Tr5 are both NMOS FETs in some embodiment,the gate G5 may be designed to have a different work function than thegate G4.

In some SRAM cell designs, the gates G4 and G5 are connected by sharinga common metal layer in their gate stacks (in these embodiments, thegate G5 is not connected to the contact 128). This might cause unbalancebetween the transistors Tr1 and Tr4, both of which are NMOS FETs, forthe following two reasons. First, the gate G1 has an end cap to the leftof the active region 102 while the gate G4 extends all the way to theactive region 110. Here, an “end cap” refers to the extension of a gatebeyond the width of the active region (e.g., extension along the “x”direction in FIG. 1B). A shorter end cap in the gate G1 typically causesan increase in the work function thereof. Second, the gates G4 and G5may have different gate stacks, such as different work function metallayers. When the gates G4 and G5 share a common metal layer, the metalelements of the two gate stacks may intermix to affect the work functionof each gate. Particularly, when the gate G5 has a lower work function,metal elements of the gate G5 migrating into the gate G4 would tend toreduce the work function of the gate G4. Either or both reasons abovewould result in a higher threshold voltage in the transistor Tr1 thanthe transistor Tr4, which ideally should match each other. Consequently,the transistor Tr4 in those 8T SRAM cells needs a higher Vccmin tooperate reliably, thereby increasing the overall Vccmin of the 8T SRAMcell. In contrast, the SRAM cell 100 of the present embodiment has thegates G4 and G5 electrically isolated to overcome the unbalance issuediscussed above.

Referring to FIG. 1B, the insulating material 112 electrically isolatesthe gates G4 and G5. The end cap of the gate G4 to the right of theactive region 108 can be adjusted to be equal to, shorter than, orlonger than the end cap of the gate G1 to the left of the active region102. This provides the flexibility of making the two gates G1 and G4(and in turn, the transistors Tr1 and Tr4) to match each other.Furthermore, the insulating material 112 prevents the metal elements ofthe gates G4 and G5 from intermixing.

There are multiple ways of isolating the gates G4 and G5. One way is todefine the gates G4 and G5 as separate gates during mask making andphotolithography. FIG. 1C illustrates a structure of the gates G4 and G5formed by this method, in accordance with one embodiment. Another way isto form a common gate and then cut the common gate into separate gatesG4 and G5. FIG. 1D illustrates the structure of the gates G4 and G5formed by this cut-gate method, in accordance with one embodiment.Various other embodiments of forming the gates G4 and G5 arecontemplated to be within the scope of the present disclosure. Each ofthe FIGS. 1C and 1D is a cross-sectional view of the SRAM cell 100 takenalong the A-A′ line of FIG. 1B.

Referring to FIG. 1C, in this embodiment, the active regions including106, 108, and 110 have fin-like structures (fins), making thetransistors Tr1 through Tr8 FinFETs. The fins 106, 108, and 110 extendupwardly from a substrate 96 and are isolated from each other by anisolation structure 98. The substrate 96 is a silicon substrate in thepresent embodiment. Alternatively, the substrate 96 may comprise anotherelementary semiconductor, such as germanium; a compound semiconductorincluding silicon carbide, gallium arsenic, gallium phosphide, indiumphosphide, indium arsenide, and/or indium antimonide; an alloysemiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP,and/or GaInAsP; or combinations thereof.

The isolation structure 98 may be formed of silicon oxide, siliconnitride, silicon oxynitride, fluoride-doped silicate glass (FSG), alow-k dielectric material, and/or other suitable insulating material.The isolation structure 98 may be shallow trench isolation (STI)features. In an embodiment, the isolation structures 98 is formed byetching trenches in the substrate 96, e.g., as part of the fin formationprocess. The trenches may then be filled with isolating material,followed by a chemical mechanical planarization (CMP) process. Otherisolation structure such as field oxide, LOCal Oxidation of Silicon(LOCOS), and/or other suitable structures are possible. The isolationstructure 98 may include a multi-layer structure, for example, havingone or more thermal oxide liner layers.

Still referring to FIG. 1C, the gates G3, G4, and G5 are disposed overthe fins 106, 108, and 110 respectively. The gate G3 includes a gatedielectric layer 150, one or more work function layer 152, and a metalfill layer 158. The gate G4 includes the gate dielectric layer 150, oneor more work function layer 154, and the metal fill layer 158. The gateG5 includes the gate dielectric layer 150, one or more work functionlayer 154, and the metal fill layer 158. Although not shown, there maybe an interfacial layer under the gate dielectric layer 150. Theinterfacial layer may include a dielectric material such as siliconoxide layer (SiO₂) or silicon oxynitride (SiON), and may be formed bychemical oxidation, thermal oxidation, atomic layer deposition (ALD),CVD, and/or other suitable dielectric. The gate dielectric layer 150 mayinclude a high-k dielectric layer such as hafnium oxide (HfO₂),zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), titanium oxide (TiO₂),yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), other suitablemetal-oxides, or combinations thereof. The gate dielectric layer 150 maybe formed by ALD and/or other suitable methods. The work function layers152, 154, and 156 may be a p-type or an n-type work function layerdepending on the respective transistor's conductivity type. The p-typework function layer comprises a metal with a sufficiently largeeffective work function, selected from but not limited to the group oftitanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru),molybdenum (Mo), tungsten (W), platinum (Pt), or combinations thereof.The n-type work function layer comprises a metal with sufficiently loweffective work function, selected from but not limited to the group oftitanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbidenitride (TaCN), tantalum silicon nitride (TaSiN), or combinationsthereof. The work function layer 150 may include a plurality of layersand may be deposited by CVD, PVD, and/or other suitable process. Themetal fill layer 158 may include aluminum (Al), tungsten (W), cobalt(Co), copper (Cu), and/or other suitable materials. The metal fill layer158 may be formed by CVD, PVD, plating, and/or other suitable processes.The gate electrode in each of the gates G3, G4, and G5 includes therespective work function layer (152, 154, or 156) and the metal filllayer 158. In the present embodiment, the gates G3 and G4 areelectrically connected by the common metal fill layer 158.

In an embodiment, the gates G3/G4 and G5 are defined as separate gatesduring photolithography which includes multiple deposition and etchingprocesses to form two separate trenches in place of the gates G3/G4 andG5. The trenches are surrounded by the insulating materials 112 and 114on their sidewalls. Subsequently, the various layers 150, 152/154/156,and 158 are deposited into the two trenches to form the gates G3, G4 andG5. Particularly, the gate dielectric layer 150 is deposited ontosidewalls of the two trenches.

Referring to FIG. 1D, in this embodiment, the gates G3/G4 and G5 areinitially connected during their formation process. Then, a cut processis performed to separate the gate G5 from the gates G3/G4, and resultsin a trench between the gates G4 and G5. Subsequently, the insulatingmaterial 112 is deposited to fill in the trench. As a result, the gatedielectric layer 150 is deposited onto the fins 106/108/110 and theisolation structure 98 but not onto the upper portion of the insulatingmaterial 112.

Referring back to FIG. 1B, the connectivity between the read port andthe write port of the SRAM cell 100 in the present embodiment isachieved by connecting the gate G5 to the S/D contact 128. Particularly,the S/D contact 128 extends into the region for the read port betweenthe gates G5 and G6. Then, the conductive feature 134 electricallyconnects the S/D contact 128 to the gate G5. The conductive feature 134may include one or more layers of conductive materials. FIGS. 1E, 1F,1G, and 1H show four different embodiments of the conductive feature134. Each of the FIGS. 1E, 1F, 1G, and 1H is a cross-sectional view ofthe SRAM cell 100 taken along the B-B′ line of FIG. 1B. Various otherembodiments of electrically connecting the S/D contact 128 to the gateG5 are contemplated to be within the scope of the present disclosure.

Referring to FIG. 1E, in this embodiment, the conductive feature 134 iselectrically connected to the upper portion of the gate G5, but not thelower portion of the gate G5. In an embodiment, the conductive feature134 may be formed as part of a butted (or shared) contact. For example,after the gates G1-G8 are formed in trenches surrounded by a dielectricgate spacer 166 which is in turn surrounded by the insulating material114, one or more etching processes are performed to etch trenches intothe insulating material 114 to expose the S/D regions of the transistorsTr1-Tr8. As part of these etching processes, an upper and side portionof the gate G5 is also exposed. Particularly, part of the gate electrodeincluding the work function layer 156 and the metal fill layer 158 isexposed. Subsequently, one or more conductive materials are filled intothe trenches to form the S/D contacts including the S/D contact 128. Theconductive feature 134 is thereby formed as part of the S/D contact 128in this process. The S/D contact 128 (and the conductive feature 134)may include a conductive barrier layer (such as TiN or TaN) and a metallayer over the conductive barrier layer. The metal layer may includealuminum (Al), copper (Cu), tungsten (W), or other suitable material.

Referring to FIG. 1F, in this embodiment, the conductive feature 134 iselectrically connected to the upper portion of the gate G5, but not thelower portion of the gate G5. In an embodiment, the conductive feature134 may be formed as a butted (or shared) contact as discussed withrespect to FIG. 1E. To further this embodiment, the etching processes donot completely remove the insulating material 114 between the S/Dcontact 128 and the gate spacer 166. In another embodiment, the S/Dcontact 128 and the conductive feature 134 may be formed in differentprocesses. For example, after the gate G5 and the S/D contact 128 havebeen formed, an etching process is performed to form a trench betweenthe gate G5 and the S/D contact 128, and to further expose a sideportion of the S/D contact 128 and a side portion of the gate electrodeof the gate G5. Then, one or more conductive materials are depositedinto the trench to form the conductive feature 134, which may include aconductive barrier layer (such as TiN or TaN) and a metal layer (such asAl, Cu, or W) over the conductive barrier layer.

Referring to FIG. 1G, in this embodiment, the conductive feature 134 iselectrically connected to a side portion of the gate electrode of thegate G5 and a sidewall of the S/D contact 128. Particularly, theconductive feature 134 is connected to both the upper and lower portionsof the sidewall of the gate electrode of the gate G5. In an exemplaryformation process, after the gate G5 and the S/D contact 128 have beenformed, an etching process is performed to form a trench between thegate G5 and the S/D contact 128. In an embodiment, the etching processis tuned to selectively remove the insulating material 114 and the gatedielectric layer 150 but not the S/D contact 128 and the gate electrode156/158. The etching process exposes a side portion of the S/D contact128 and a sidewall of the gate electrode of the gate G5. Particularly,the etching process fully exposes the sidewall of the work functionlayer 156 of the gate G5. Then, one or more conductive materials aredeposited into the trench to form the conductive feature 134, which mayinclude a conductive barrier layer (such as TiN or TaN) and a metallayer (such as Al, Cu, or W) over the conductive barrier layer.

Referring to FIG. 1H, in this embodiment, the conductive feature 134includes conductive plugs (or vias) 170 and 172 and a conductive wire174. The plugs 170 and 172 are disposed over the S/D contact 128 and thegate G5 respectively. The wire 174 electrically connects the plugs 170and 172. Each of the plugs 170 and 172 may comprise one or moreconductive barrier layers (such as TiN or TaN) and a metal layer (suchas Al, Cu, or W). The wire 174 may comprise one or more conductivebarrier layers (such as TiN or TaN) and a metal layer (such as Al, Cu,or W). The plugs 170 and 172 may be formed by etching holes into theinsulating material 114 above the S/D contact 128 and the gate G5, andthen filling the holes with one or more conductive materials. The wire174 may be formed by etching a trench in the insulating material 114,and filling the trench with one or more conductive materials. The plugs170/172 and the wire 174 may be formed by single damascene processes,dual damascene processes, or other suitable processes.

FIG. 2A illustrates a top view of an SRAM cell 200 in an embodiment.FIG. 2B shows the logic diagram of the SRAM cell 200 in an embodiment.The SRAM cell 200 is essentially the same as the SRAM cell 100 shown inFIG. 1B, except that the read port transistors are flipped in terms oftheir functions. Referring to FIG. 2A, the S/D contact 128 iselectrically connected to the gate G6 in the SRAM cell 200 instead ofthe gate G5 in the SRAM cell 100. In the SRAM cell 200, the S/D contact136 serves as the RBL terminal and the S/D contact 138 serves as theVSS2 terminal. The conductive feature 134 may take the form of any ofthe embodiments shown in FIGS. 1E-1H, or may take other forms. Otheraspects of the SRAM cell 200 are the same as the SRAM cell 100. Lookingat the SRAM cells 100 and 200 from a different perspective, the SRAMcell 200 is the same as the SRAM cell 100 with the transistor Tr5 placedabove (along the “y” direction) the transistor Tr6.

FIG. 3A illustrates a top view of an SRAM cell 300 in an embodiment.FIG. 3B shows the logic diagram of the SRAM cell 300 in one embodiment.The SRAM cell 300 is essentially the same as the SRAM cell 100 shown inFIG. 1B, except that the read port is placed on the left side of thewrite port and the gate G5 is connected to the S/D contact 122 in theSRAM cell 300 rather than the S/D contact 128 as in SRAM cell 100.Referring to FIG. 3A, the S/D contact 122 extends into the region forthe read port and between the gates G5 and G6. The conductive feature134 electrically connects the S/D contact 122 to the gate G5. Theconductive feature 134 may take the form of any of the embodiments shownin FIGS. 1E-1H, or may take other forms. Other aspects of the SRAM cell200 and the same as the SRAM cell 100.

FIG. 4A shows a top view of an SRAM cell 400 constructed according toaspects of the present disclosure. The SRAM cell 400 also comprises thetransistors Tr1 through Tr8, but the placement of the transistors isdifferent from that of the SRAM cell 100. Particularly, the write portin the SRAM cell 400 is split into a left portion (write port left) anda right portion (write port right). The left portion comprises thetransistors Tr1, Tr2, and Tr7. The right portion comprises thetransistors Tr3, Tr4, and Tr8. The read port is placed between the leftand right portions of the write port. Both the gates G1 and G4 have anend cap, and can be designed and manufactured to allow the transistorsTr1 and Tr4 to match each other. Similar to the SRAM cells 100, 200, and300, the SRAM cell 400 also provides benefits of balanced transistors inthe write port and reduced Vccmin. Many aspects of the SRAM cell 400 arethe same as those of the SRAM cell 100. Some differences are discussedbelow.

The S/D contact 122 is electrically connected to the gate G3 throughconductive features 180 and 182. Particularly the conductive feature 180is routed above the active region 110 without electrically contactingthe S/D regions of the transistors Tr5 and Tr6. The gates G1, G2, G5 areelectrically connected, for example by sharing a common metal layer inthe respective gates. The gate G5 further extends into the right portionof the write port. The extension of the gate G5 is referred to as theconductive feature 184. The conductive feature 184 is electricallyconnected to the S/D contact 128 through a conductive feature 186. Theconductive feature 186 may be similar to the conductive feature 134 invarious embodiments. In the embodiment shown in FIG. 4A, the SRAM cell400 includes a remnant gate feature 188 which is a residue after somecut-gate process that separates the gates G7, G6, and G3/G4.

FIG. 4B illustrates the conductive features 180 and 182 according to anembodiment. Referring to FIG. 4B, the conductive features 180 and 182are in the form of local interconnection. The conductive feature 180 isdisposed over the S/D contact 122, the conductive feature 182 isdisposed over the gate G3, and the conductive features 180 and 182 aredirectly connected. In an embodiment, the process of forming theconductive features 180 and 182 includes etching trenches in theinsulating material 114 to define the shape of the local interconnectionand to expose at least the top surface of the S/D contact 122 and thetop surface of the gate G3. The process further includes depositing oneor more conductive materials into the trench. The one or more conductivematerials may include conductive barrier layer(s) (such as TiN or TaN)and a metal layer (such as Al, Cu, or W). In this embodiment, theconductive features 180 and 182 are at the same fabrication level.

In another embodiment, the conductive features 180 and 182 may be in theform of interconnection at upper metal layers rather than localinterconnection. For example, each of the conductive features 180 and182 may include one or more vias and one or more metal wires, and theconductive features 180 and 182 may be at the same metal layer ordifferent metal layers.

Although not intended to be limiting, the present disclosure providesmany benefits. For example, various designs and layouts of 8T SRAM cellaccording to the present disclosure provide balanced transistors,especially balanced pull-down transistors, in the write port of therespective SRAM cell. The balanced transistors allow the minimumoperation voltage (Vccmin) of the SRAM cells to be reduced, therebyreducing the power consumption thereof. Even though 8T SRAM cells areused as examples, the present disclosure are not limited to 8T SRAMcells, but are applicable to other types of SRAM cells and circuits ingeneral.

In one exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes first, second,third, fourth, and fifth active regions arranged in order from first tofifth along a first direction. The first, second, third, and fourthactive regions comprise channel regions and source/drain (S/D) regionsof first, second, third, and fourth transistors respectively, and thefifth active region comprises channel regions and S/D regions of fifthand sixth transistors. The semiconductor device further includes first,second, third, fourth, fifth, and sixth gates oriented along the firstdirection. The first through sixth gates are configured to engage thechannel regions of the first through sixth transistors respectively. Thefirst and second gates are electrically connected. The third and fourthgates are electrically connected. The semiconductor device furtherincludes one or more first conductive features that electrically connectone of the S/D regions of the first transistor, one of the S/D regionsof the second transistor, and the third gate. The semiconductor devicefurther includes one or more second conductive features thatelectrically connect the second gate, one of the S/D regions of thethird transistor, one of the S/D regions of the fourth transistor, andthe fifth gate.

In an embodiment of the semiconductor device, each of the first throughfifth active regions comprises a fin, and each of the first throughsixth transistors is a FinFET. In an embodiment of the semiconductordevice, the first and fourth transistors are of a first conductivitytype, the second and third transistors are of a second conductivity typeopposite to the first conductivity type, and the fifth and sixthtransistors are of a same conductivity type.

In an embodiment of the semiconductor device, the first active regionfurther comprises a channel region and S/D regions of a seventhtransistor, and the fourth active region further comprises a channelregion and S/D regions of an eighth transistor. In a further embodiment,the semiconductor device includes seventh and eighth gates, wherein theseventh and eighth gates are configured to engage the channel regions ofthe seventh and eighth transistors respectively.

In an embodiment of the semiconductor device, the one or more secondconductive features comprise an S/D contact feature disposed over theS/D region of the fourth transistor; and a butted contact connecting theS/D contact feature to the fifth gate. In a further embodiment, the S/Dcontact feature is also disposed over the S/D region of the thirdtransistor. In an embodiment of the semiconductor device, the one ormore second conductive features comprise an S/D contact feature disposedover the S/D region of the fourth transistor; and a conductive featureelectrically connecting the S/D contact feature to at least a lowerportion of the fifth gate.

In an embodiment of the semiconductor device, the one or more secondconductive features comprise an S/D contact feature disposed over theS/D region of the fourth transistor; a first plug disposed over the S/Dcontact feature; a second plug disposed over the fifth gate; and aconductive wire connecting the first and second plugs.

In an embodiment of the semiconductor device, the first, second, andsixth gates are arranged along a straight line, and the third, fourth,and fifth gates are arranged along another straight line.

In an embodiment of the semiconductor device, the first, second, andfifth gates are arranged along a straight line, and the third, fourth,and sixth gates are arranged along another straight line.

In another exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes first, second,third, fourth, and fifth semiconductor fins oriented lengthwise along afirst direction and arranged in order from first to fifth along a seconddirection perpendicular to the first direction. The first, second,third, and fourth semiconductor fins comprise channel regions of first,second, third, and fourth FinFET transistors respectively, and the fifthsemiconductor fin comprises channel regions of fifth and sixth FinFETtransistors. The semiconductor device further includes first, second,third, fourth, fifth, and sixth gate stacks oriented along the seconddirection, wherein the first through sixth gate stacks are disposed overthe channel regions of the first through sixth transistors respectively.The semiconductor device further includes a first plurality ofconductive features that electrically connect a source/drain (S/D)region of the first transistor, a S/D region of the second transistor,and the third gate stack. The semiconductor device further includes asecond plurality of conductive features that electrically connect thesecond gate stack, a S/D region of the third transistor, a S/D region ofthe fourth transistor, and the fifth gate stack. In the semiconductordevice, the first and second gate stacks are electrically coupled, thethird and fourth gate stacks are electrically coupled, the first andsecond FinFETs are of opposite conductivity types, the third and fourthFinFETs are of opposite conductivity types, and the fifth and sixthFinFETs are of a same conductivity type.

In an embodiment of the semiconductor device, the second plurality ofconductive features include a shared contact disposed over the S/Dregion of the third FinFET, the S/D region of the fourth FinFET, and thefifth gate stack. In a further embodiment, the shared contact isdisposed over a top surface of the fifth gate stack. In a furtherembodiment, the shared contact is disposed over a side and conductiveportion of the fifth gate stack.

In an embodiment of the semiconductor device the second plurality ofconductive features include a contact feature disposed over the S/Dregion of the fourth FinFET; a first via disposed over the contactfeature; and a second via disposed over the fifth gate stack, whereinthe first and second vias are electrically connected.

In another exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes first, second,third, and fourth transistors arranged in order from first to fourthalong a first direction. The first and fourth transistors are NMOS FET.The second and third transistors are PMOS FET. Each of the first throughfourth transistors comprises a channel region, two source/drain (S/D)regions, and a gate stack over the respective channel region. Thesemiconductor device further includes fifth and sixth transistorsbetween the second and third transistors. The fifth and sixthtransistors are of a same conductivity type. Each of the fifth and sixthtransistors comprises a channel region, two source/drain (S/D) regions,and a gate stack over the respective channel region. The gate stacks ofthe first, second, and fifth transistors, one of the S/D regions of thethird transistor, and one of the S/D regions of the fourth transistorare electrically connected. The gate stacks of the third and fourthtransistors, one of the S/D regions of the first transistor, and one ofthe S/D regions of the second transistor are electrically connected.

In an embodiment of the semiconductor device, the fifth and sixthtransistors are PMOS FETs. In another embodiment of the semiconductordevice, the fifth and sixth transistors are NMOS FETs. In yet anotherembodiment of the semiconductor device the first through sixthtransistors are FinFETs.

In another exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes first, second,third, and fourth active regions arranged in order from first to fourthalong a first direction, wherein the first, second, third, and fourthactive regions comprise channel regions and source/drain (S/D) regionsof first, second, third, and fourth transistors respectively. The firstand fourth transistors are of a first conductivity type, and the secondand third transistors are of a second conductivity type opposite thefirst conductivity type. The semiconductor device further includes afifth active region between the second and third active regions, whereinthe fifth active region comprises channel regions and S/D regions offifth and sixth transistors that are of same conductivity type. Thesemiconductor device further includes first, second, third, fourth,fifth, and sixth gates, wherein the first through sixth gates aredisposed over the channel regions of the first through sixth transistorsrespectively, wherein the first, second, and fifth gates areelectrically connected, and the third and fourth gates are electricallyconnected. The semiconductor device further includes one or more firstconductive features that electrically connect one of the S/D regions ofthe first transistor, one of the S/D regions of the second transistor,and the third gate. The semiconductor device further includes one ormore second conductive features that electrically connect the fifthgate, one of the S/D regions of the third transistor, and one of the S/Dregions of the fourth transistor.

In an embodiment of the semiconductor device, the one or more firstconductive features include a contact feature disposed over the one ofthe S/D regions of the second transistor and a local interconnectdisposed directly over the contact feature and the fifth gate. In anembodiment of the semiconductor device, the one or more first conductivefeatures include a conductive feature that is disposed over andinsulated from one of the S/D regions of the fifth and sixthtransistors. In an embodiment of the semiconductor device, the first,second, and fifth gate share a common metal layer.

In an embodiment of the semiconductor device, the first active regionfurther comprises a channel region and S/D regions of a seventhtransistor, and the fourth active region further comprises a channelregion and S/D regions of an eighth transistor. In a further embodiment,the semiconductor device further includes seventh and eighth gates,wherein the seventh and eighth gates are disposed over the channelregions of the seventh and eighth transistors respectively.

In an embodiment of the semiconductor device, each of the first throughsixth transistors are FinFETs. In an embodiment of the semiconductordevice, the fifth and sixth transistors share a common S/D region. In anembodiment of the semiconductor device, the fifth and sixth transistorsare of the first conductivity type. In another embodiment of thesemiconductor device, the fifth and sixth transistors are of the secondconductivity type.

In another exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes first, second,third, and fourth FinFETs arranged in order from first to fourth along afirst direction. The first and fourth transistors are of a firstconductivity type, the second and third transistors are of a secondconductivity type opposite the first conductivity type, and each of thefirst through fourth transistors comprises a channel region, twosource/drain (S/D) regions, and a gate stack over the respective channelregion. The semiconductor device further includes fifth and sixthFinFETs between the second and third FinFETs. The fifth and sixthFinFETs are of a same conductivity type, and each of the fifth and sixthtransistors comprises a channel region, two source/drain (S/D) regions,and a gate stack over the respective channel region. In thesemiconductor device, the gate stacks of the first, second, and fifthFinFETs, one of the S/D regions of the third FinFET, and one of the S/Dregions of the fourth FinFET are electrically connected; the gate stacksof the third and fourth FinFETs, one of the S/D regions of the firstFinFET, and one of the S/D regions of the second FinFET are electricallyconnected; and the fifth and sixth FinFETs share a common S/D region.

In an embodiment of the semiconductor device, the first conductivitytype is n-type and the second conductivity type is p-type. In anembodiment of the semiconductor device, the fifth and sixth FinFETs areof the first conductivity type. In another embodiment of thesemiconductor device, the fifth and sixth FinFETs are of the secondconductivity type. In an embodiment of the semiconductor device, thechannel regions of the fifth and sixth FinFETs are in a same fin. In anembodiment of the semiconductor device, the gate stacks of the first,second, and fifth FinFETs share a common metal layer.

In another exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes first, second,third, fourth, fifth, and sixth transistors. The first and fourthtransistors are NMOS FET, the second and third transistors are PMOS FET,and the fifth and sixth transistors are of a same conductivity type.Each of the first through sixth transistors comprises a channel region,two source/drain (S/D) regions, and a gate stack over the respectivechannel region. In the semiconductor device, the channel regions of thefirst through fifth transistors are arranged in order from first tofifth along a first direction; the gate stacks of the first, second, andfifth transistors, one of the S/D regions of the third transistor, andone of the S/D regions of the fourth transistor are electricallyconnected; and the gate stacks of the third and fourth transistors, oneof the S/D regions of the first transistor, and one of the S/D regionsof the second transistor are electrically connected.

In an embodiment of the semiconductor device, the channel regions of thefifth and sixth transistors are aligned along a second directionperpendicular to the first direction. In an embodiment of thesemiconductor device, the fifth and sixth transistors are PMOS FETs. Inan embodiment of the semiconductor device, the first through sixthtransistors are FinFETs.

The foregoing outlines features of several embodiments so that thosehaving ordinary skill in the art may better understand the aspects ofthe present disclosure. Those having ordinary skill in the art shouldappreciate that they may readily use the present disclosure as a basisfor designing or modifying other processes and structures for carryingout the same purposes and/or achieving the same advantages of theembodiments introduced herein. Those having ordinary skill in the artshould also realize that such equivalent constructions do not departfrom the spirit and scope of the present disclosure, and that they maymake various changes, substitutions, and alterations herein withoutdeparting from the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor device, comprising: first,second, third, and fourth active regions arranged in order from first tofourth along a first direction, wherein the first, second, third, andfourth active regions comprise channel regions and source/drain (S/D)regions of first, second, third, and fourth transistors respectively,the first and fourth transistors are of a first conductivity type, andthe second and third transistors are of a second conductivity typeopposite the first conductivity type; a fifth active region between thesecond and third active regions, wherein the fifth active regioncomprises channel regions and S/D regions of fifth and sixth transistorsthat are of same conductivity type; first, second, third, fourth, fifth,and sixth gates, wherein the first through sixth gates are disposed overthe channel regions of the first through sixth transistors respectively,wherein the first, second, and fifth gates are electrically connected,and the third and fourth gates are electrically connected; one or morefirst conductive features that electrically connect one of the S/Dregions of the first transistor, one of the S/D regions of the secondtransistor, and the third gate; and one or more second conductivefeatures that electrically connect the fifth gate, one of the S/Dregions of the third transistor, and one of the S/D regions of thefourth transistor.
 2. The semiconductor device of claim 1, wherein theone or more first conductive features include: a contact featuredisposed over the one of the S/D regions of the second transistor; and alocal interconnect disposed directly over the contact feature and thefifth gate.
 3. The semiconductor device of claim 1, wherein the one ormore first conductive features include a conductive feature that isdisposed over and insulated from one of the S/D regions of the fifth andsixth transistors.
 4. The semiconductor device of claim 1, wherein thefirst, second, and fifth gate share a common metal layer.
 5. Thesemiconductor device of claim 1, wherein the first active region furthercomprises a channel region and S/D regions of a seventh transistor, andthe fourth active region further comprises a channel region and S/Dregions of an eighth transistor.
 6. The semiconductor device of claim 5,further comprising seventh and eighth gates, wherein the seventh andeighth gates are disposed over the channel regions of the seventh andeighth transistors respectively.
 7. The semiconductor device of claim 1,wherein each of the first through sixth transistors are FinFETs.
 8. Thesemiconductor device of claim 1, wherein the fifth and sixth transistorsshare a common S/D region.
 9. The semiconductor device of claim 1,wherein the fifth and sixth transistors are of the first conductivitytype.
 10. The semiconductor device of claim 1, wherein the fifth andsixth transistors are of the second conductivity type.
 11. Asemiconductor device, comprising: first, second, third, and fourthtransistors arranged in order from first to fourth along a firstdirection, wherein the first and fourth transistors are NMOS FET, thesecond and third transistors are PMOS FET, and each of the first throughfourth transistors comprises a channel region, two source/drain (S/D)regions, and a gate stack over the respective channel region; and fifthand sixth transistors between the second and third transistors, whereinthe fifth and sixth transistors are of a same conductivity type, andeach of the fifth and sixth transistors comprises a channel region, twosource/drain (S/D) regions, and a gate stack over the respective channelregion, wherein the gate stacks of the first, second, and fifthtransistors, one of the S/D regions of the third transistor, and one ofthe S/D regions of the fourth transistor are electrically connected, andwherein the gate stacks of the third and fourth transistors, one of theS/D regions of the first transistor, and one of the S/D regions of thesecond transistor are electrically connected.
 12. The semiconductordevice of claim 11, wherein the fifth and sixth transistors are PMOSFETs.
 13. The semiconductor device of claim 11, wherein the fifth andsixth transistors are NMOS FETs.
 14. The semiconductor device of claim11, wherein the first through sixth transistors are FinFETs.
 15. Asemiconductor device, comprising: first, second, third, and fourthFinFETs arranged in order from first to fourth along a first direction,wherein the first and fourth transistors are of a first conductivitytype, the second and third transistors are of a second conductivity typeopposite the first conductivity type, and each of the first throughfourth transistors comprises a channel region, two source/drain (S/D)regions, and a gate stack over the respective channel region; and fifthand sixth FinFETs between the second and third FinFETs, wherein thefifth and sixth FinFETs are of a same conductivity type, and each of thefifth and sixth transistors comprises a channel region, two source/drain(S/D) regions, and a gate stack over the respective channel region,wherein the gate stacks of the first, second, and fifth FinFETs, one ofthe S/D regions of the third FinFET, and one of the S/D regions of thefourth FinFET are electrically connected, wherein the gate stacks of thethird and fourth FinFETs, one of the S/D regions of the first FinFET,and one of the S/D regions of the second FinFET are electricallyconnected, and wherein the fifth and sixth FinFETs share a common S/Dregion.
 16. The semiconductor device of claim 15, wherein the firstconductivity type is n-type and the second conductivity type is p-type.17. The semiconductor device of claim 15, wherein the fifth and sixthFinFETs are of the first conductivity type.
 18. The semiconductor deviceof claim 15, wherein the fifth and sixth FinFETs are of the secondconductivity type.
 19. The semiconductor device of claim 15, wherein thechannel regions of the fifth and sixth FinFETs are in a same fin. 20.The semiconductor device of claim 15, wherein the gate stacks of thefirst, second, and fifth FinFETs share a common metal layer.